The design of biocular optics for projected displays must take into account a variety of factors to display the image as desired. It must be color corrected for the desired wavelengths, must maintain imaging and brightness performance over its entire eye motion box (EMB), and when both eyes are in the exit aperture, it must have the specified level of parallax between the left and right eye images.
Biocular displays are typically designed to project a single object as a stereoscopic image pair, which will appear from a defined apparent distance. When the angles of incidence at the observer's eyes are parallel between the left and right eyes, the image will appear to come from infinity. If the azimuth angle between the left and right eye views of object points varies over the image, the object scene will appear to be on a surface with a specific shape. Because the eyes are separated in the horizontal plane, there is no real interpretation to an elevation angular difference between the left and right eye views. Similarly there are no real objects that require the left and right eyes to diverge (point outwards) in order to view stereoscopically, as such object would (via parallax) appear to be located ‘beyond’ infinity. In both cases such a projection will result in double images (failure to achieve stereopsis), eyestrain, and/or discomfort.
A further issue results when the apparent location of viewed image points moves as the eyes move laterally in the EMB, meaning that the angle of projection of an object point varies as a function of lateral eye position. In such cases when this aberration is excessive and because an observer's head and eyes are in constant motion, the image is often interpreted as grossly distorted, disorienting, and ‘swimming’.
As shown in FIG. 4, this situation is well known in the art. As illustrated, a rightward translation of the eyes would cause the apparent location of the images to also shift to the right. Because this is in the opposite direction to the visual effect experienced when viewing stationary objects directly, the image would appear to be rapidly moving.
All of these negative issues result from parallax errors, an uncorrected difference in the intended angle of light rays from the same object point when projected into different locations in the system exit optical aperture. For large aperture optics (and therefore low F/Number), correction over the whole aperture presents a challenge.
Correction of ray angles for all possible aperture positions typically requires an increased number of lenses or other optical design degrees of freedom such as the uses of aspheric or diffractive surfaces. These measures add cost, complexity, and also tend to increase the physical size as more lenses take up more room. The requirement for precise parallax correction also limits the ability to implement other potentially desirable attributes such as higher resolution, multicolor projection, and wider fields of view.
Human eye pupils have a diameter varying between 4 and 7 mm, depending on the magnitude of background light, direct illumination and psychological-physiological effects. Artificial imagery created by viewing devices and projectors is usually projected to large apertures covering sufficient area where eyes of viewers may be located. The typical imaging aperture may occupy an area of 50 cm2, a factor over fifty relative to a pair of the broadest pupils. Therefore, savings by a similar factor of the projection luminance may be rendered once the image is projected only into sub-apertures coinciding with the pupils. Such luminance saving induces electric power saving which is particularly beneficial in portable, airborne and spaceborne applications. Furthermore, projection of an image to a large aperture permits viewing by all incident viewers, whereas projection to the sub-apertures is exclusive to a particular viewer.
The design of biocular optics for projected displays must be color corrected for the desired wavelengths, must maintain imaging and brightness performance over its entire eye motion box (EMB), and when both eyes are in the exit aperture, it must have the specified level of parallax between the left and right eye images. In addition the apparatus must be designed as a telecentric system stemming from the fact that the viewer eyes are directed at a known target, for instance a screen or a combiner. Then it also possesses the advantage wherein third order aberrations such as coma and astigmatism can be canceled. With respect to parallax, rays of parallel angle of incidence at the observer's left and right eyes appear to come from infinity, whereas azimuth angle between the left and right eye makes the image reside on a surface with a specific shape. When the apparent location of viewed image points moves as the eyes move laterally in the angle of projection of an object point varies as a function of lateral eye position. If the optical aberration is excessive and because an observer's head and eyes are in constant motion, the image in this case becomes grossly distorted. The uncorrected difference in the intended angle of light rays emanating from the same object point, when projected into different locations in the system exit aperture, result in parallax errors.
Without the need to correct for parallax, an avionic HUD optic need only be corrected over any F/12 to F/25 sub aperture, the aperture limit set by an observer's single eye as opposed to the whole. As most HUD's are monochromatic this would be relatively straightforward. Typically, a biocular HUD optic operates over and requires parallax correction for an F/0.8 to F/1.5 relative aperture, which is quite challenging. A typical specification for parallax in an avionic HUD application is less than 1.5 milliradians in the central portion of the field of view (FOV) and less than 6 milliradians in the outer portions of the FOV.
U.S. Pat. No. 6,874,894, issued to Kitamura on Apr. 5, 2005, discloses a projector equipped with a DMD (digital micro-mirror device). The projector is provided where an image is generated by an image display device receiving an image data from a personal computer or a video camera and then is projected on a screen. The projector hence includes an optical system for projection of images.
U.S. Pat. No. 5,978,128, issued to Yoon on Nov. 2, 1999 discloses a deformable mirror device (DMD) for changing a proceeding path of an incident light, and more particularly, to a deformable mirror device having an improved structure so that the path of light can be easily changed with a low driving voltage.
U.S. Pat. No. 5,805,119, issued to Erskine et al. on Sep. 8, 1998, discloses projected displays suitable for use in motor vehicles. It reflects information the vehicle operator needs off of a half mirror or the windshield and projects that information at a distance in front of the vehicle. Sometimes heads-up displays are used in automotive applications such that the image is projected up and reflected off of the vehicle front windshield to appear at a distance in front of the driver. In such cases, the front windshield is used as a combiner allowing the head-up display image to appear together with the view through the front windshield. With a heads-up display in a vehicle, the driver does not have to adjust his/her eyes from the road to read information such as vehicle speed, which is normally displayed in the vehicle instrument panel.
U.S. Pat. No. 7,271,960, issued to Stewart on Sep. 18, 2007, discloses an integrated heads-up-display (HUD) device including a housing that houses an active-matrix image projecting system and its accompanying electronics. An optical combiner is connected to the housing via a retractable arm attached at one end to the main body and holds the HUD optical panel at its other end. A telescopically retractable arm allows the assembly to extend or retract for a desirable combiner height. Further, the LED backlighting array is placed within a reflective light chamber, which is lined with a safe, lightweight, low cost, specular, reflective sheet for high reflectivity
U.S. Pat. No. 7,413,309, issued to Whitehead et al. on Aug. 19, 2008, discloses a display having a screen, which incorporates a light modulator. The screen may be a front projection screen or a rear-projection screen. Elements of the light modulator may be controlled to adjust the intensity of light emanating from corresponding areas on the screen. The display may provide a high dynamic range.
U.S. Pat. No. 7,405,856 discloses display systems and the preferred embodiment relates to a display system with clock-dropping to compensate for lamp variations.
U.S. Pat. No. 7,377,652, issued to Whitehead et al. on May 27, 2008, discloses a display, which has a screen, which incorporates a light modulator. The screen may be a front projection screen or a rear-projection screen. Elements of the light modulator may be controlled to adjust the intensity of light emanating from corresponding areas on the screen. The display may provide a high dynamic range.
None of the above-referenced devices discloses or suggest, either alone or in combination with one another, the invention disclosed herein.